This invention relates to cellular radio systems, and, in particular, relates to reducing intermodulation products when selecting radio channels in a dynamic channel assignment system.
Cellular radio service is expanding at an explosive rate and is almost ubiquitous now. With cellular radio service, a predetermined radio frequency spectrum is allocated to carry the communication between a user""s cellular telephone and the service provider""s base station (the gateway into the cellular switching network). The spectrum is divided into frequency channels, commonly referred to as xe2x80x9cchannel numbers,xe2x80x9d and are reused by base stations within a service provider""s area. The greater the reuse of frequency channels, the greater the number of cellular radio subscribers that can be simultaneously served. However, one frequency channel cannot be used by two adjacent base stations because such uses cause interference. While reusing frequency channels more often increases the frequency spectrum efficiency, it also increases the resulting interference. Each factor is balanced against the other in order to achieve a division of the radio spectrum to provide service for more customers with little or no extra cost to the service provider.
Cellular and Personal Communications Services (PCS) technologies that are based upon Time Division Multiple Access (TDMA) and Frequency Division Multiple Access (FDMA) require some form of channel assignment scheme to allocate frequency spectrum among mobile subscriber units. In some prior cellular radio systems, channel assignment was predicated on a fixed channel reuse plan. Such an approach is often referred to as a xe2x80x9cfixed channel assignmentxe2x80x9d (FCA) and is known to satisfactorily serve uniformly heavy traffic.
When the traffic pattern is non-uniform, however, a channel assignment scheme that assigns channels dynamically can potentially serve mobile subscriber units more efficiently. While being straight-forward to implement, FCA requires elaborate base station engineering and manual engineering to install a FCA-based cellular radio system. Therefore, frequency spectrum management in cellular radio systems is migrating toward dynamic channel assignment (DCA) schemes. (DCA is also known in the art as xe2x80x9cFlexible Channel Allocationxe2x80x9d and xe2x80x9cAdaptive Channel Allocationxe2x80x9d.)
In order to improve the efficiency of a base station""s utilization of frequency spectrum, it is advantageous to automate channel allocation (which is represented by a channel number comprising a frequency pair assigned to a transmitter and a receiver) within the base station. However, channels are reused by a plurality of base stations and a 1s mobile subscriber unit communicating with its serving base station experiences interference from other base stations that are using the same channel. In order to ameliorate the effects of such interference, an RF monitoring receiver, typically situated at the serving base station, measures the level of interference associated with the channel when the channel is idle at the serving base station, before allocating the channel. If a measured level of interference on both the reverse link (mobile subscriber unit to base station) and the forward link (base station to mobile subscriber unit) both do not exceed predetermined thresholds, the channel is assigned to a call between the serving base station and the mobile subscriber unit.
Channel interference has both a long-term (time duration) interference component and a short-term interference component. The long-term interference component has slow variations, while the short-term interference component has fast variations. Causes for the long-term interference component include terrain features and system engineering considerations such as base station layout, antenna types and antenna configurations. Causes for the short-term interference component include traffic patterns and shadow fading. Shadow fading results from large terrain obstructions and buildings blocking signal paths as the mobile subscriber unit traverses the coverage area that is xe2x80x9cshadowedxe2x80x9d by an obstruction or building. Because the mobile subscriber unit passes through the affected region in a short period of time, the associated effects has fast variations.
A long-term process for measuring and analyzing long-term variations of interference ameliorates the long-term interference component by ordering the allocated frequency spectrum of each antenna sector of each cell based on a moving average of interference measurements to form a long-term list of usable channels. In order to form the initial long-term channel list, the entire frequency spectrum assigned to the wireless communications system is divided into spectral partitions wherein a spectral partition is associated with each antenna sector. The spectral partition is determined by assuming a channel set in which the members (channels) are uniformly distributed. Spectral partitions are reused so that a plurality of antenna sectors are associated with the same spectral partition in accordance with the frequency reuse factor. Typically, each spectral partition is larger than is used for a fixed channel assignment scheme, so that a greater number of candidate channels can be processed for further consideration.
A short-term process for measuring and analyzing short-term variations of interference ameliorates the short-term interference component by ordering candidate channels from the long-term list according to instantaneous interference measurements to form a short-term list. The short-term process adapts to fast (short-term) variations. The serving base station assigns a channel from a subset of the short-term list. The subset contains the xe2x80x9cbestxe2x80x9d channels having the smallest level of interference on the forward and the reverse radio channels. If all channels of the long-term list are either unused because the associated interference level exceeds a predetermined threshold or because all channels are assigned, then channel blocking occurs. Thus, increasing the size of the long-term list by effectively controlling associated interference levels increases the call capacity (and consequently the number of cellular radio subscribers) that can be supported by a cellular radio system. Any method or apparatus that increases the long-term list for dynamic channel assignment schemes is important to the economic competitiveness of the cellular radio industry.
The present invention combines the formation of a non-homogeneous channel set with a dynamic channel assignment algorithm for use by a base station in a cellular radio system. The present invention constructs a non-homogeneous, long term channel set from which channels are processed by a dynamic channel assignment algorithm to determine the current interference levels. Thus, channels having the lowest measured levels of interference are assigned to calls that are served by the cellular radio system. This synergistic combination increases the number of candidate channels with essentially the same level of interference that is experienced with respect to a homogeneous channel set in conjunction with a dynamic channel assignment. Hence, the number of subscribers that can be served by the cellular radio system is increased.
The present invention constructs a modified channel set in which constituent channels are prioritized according to a long-term process. A selected subset of the prioritized channel set is ordered according to a short-term process. Based upon the ordering, the present invention subsequently selects a channel for a call that is served by the cellular radio system.